Re: CBM-II Colour mod (was: Software for MS-DOS 1.25)

The reason for the 63* CRTC chip is due to pin and register compatibility with the 6545 and should be a drop-in replacement. I purchased a few to try on my ColourPET. If I do make a CBM-II replacement board my goal will be to produce something that has a reduced chip count and smaller form factor, while using through-hole parts wherever possible, at least as a start. After that I would be open to expanding the features. I do think it would be cool to make it both "B" and "P" series compatible. I thought about using dual-port ram for VIC-II access (in BANK15) but not sure about supporting BANK 0 access for graphics. But perhaps if all of BANK15 is RAM then a graphics screen could possibly be set up there. Since P500 had little or no software, full compatibility is not a big concern.
For PET compatibility we can always use the "8432" software, but if we could make the machine compatible without adding a lot of chips I'd be ok with that. Adding V20 would be the same. Provided the resulting board is smaller in the end.
Adding a VIC-1 would be interesting. A long time ago I thought about making a combination VIC-20/C64 machine, but that would be much easier due to the similarities of those machines. Probably not feasible in a CBM-II though.
Adding a C128-style VDC should be fairly trivial since it has it's own local RAM and needs only a couple registers mapped into the CPU space.Actually the 6545 can also be designed with local RAM as seen in the "C64 PLUS with 80 column" design. Local RAM means you can have lots of video ram that doesn't take up main cpu space but comes at the expense of access speed. For 6545/6345 I think a sliding window into video ram would be a better solution. You could have 32K (or more) video ram visible to the CRTC at all times and still have a small 2K window at it's usual address.
I wish the CBM-II had left more empty space for video ram. The CRTC can do interlace and I have actually made a 40 column by 50 line display. If it had room for 4K video ram we could have done 80x50.
Steve

      From: Mia Magnusson <mia@plea.se>
 To: cbm-hackers@musoftware.de 
 Sent: Monday, November 6, 2017 10:25 AM
 Subject: Re: CBM-II Colour mod (was: Software for MS-DOS 1.25)
   
Den Mon, 6 Nov 2017 09:38:44 +0100 skrev Konrad B
<konrad0x42@gmail.com>:
> 2017-11-06 2:30 GMT+01:00 Steve Gray <sjgray@rogers.com>:
> > In terms CRTC I'd go with the HD63B45P as it supports split-screen
> > windows and smooth scrolling.
> 
> Or is it about HD68B45SP (aka HD46505SP-2) ?  Didn't know about this.
> Nice - I ordered them already few days ago (with some HD63B03-s for
> the Sharp CE516P plotter, but they are 680X family MPUs of course;
> different story...).

63* is Hitachi, in general they have some nice features which I guess
they couldn't advertise because of their contract with Motorola.
Atleast 6309 has some nice features that 6809 lacks. Compare with NEC
V20 v.s. Intel 8088 where it was just a very well known rumor back
in the day that told us that V20 were faster than 8088, but it was
never advertised.

> >Adding a VIC-II would be problematic since it will limit the CPU to
> >1MHz.
> 
> Possible solution, this is for the C64 though, I do not remember it
> Kisiel ever tested it with the old boards (once I gave him about 10
> different HMOS-2 VIC-s for the experiments, so possibly he focused on
> the 64E):
> http://wiki.projekt64.filety.pl/doku.php?do=show&id=projekt64/turbo

Also on this list we had a discussion where I suggested something like
this, but replacing the motherboard RAM with buffers, and I was told
that 64 Ultimate 2 already does that.

It's rather easy to let VIC access ram through buffers/latches so even
though VIC uses 1MHz cycles each read is only using one cycle of a
faster memory bus, and the read result is latched until VIC has had the
time to actually process the result of the read. If some kind of modern
programmable logic should do this it might also be used for writes to
VIC (and other 1MHz I/O chips) registers.

Sorry if this is an extreme version of feature creep, but if it's
anyway going to be a new board with a 6502, hardware emulation of 6509
features, VIC-II, SID, CRTC and a lot of RAM, it might aswell be
compatible with C64 and maybe even C128. That would require emulation
of the I/O ports on 6510 (simple, and parts of that hardware can also
be used for 6509 emulation) and maybe the C128 MMU.

That way one board could be compatible with all PET, all CBM-II, C128,
C64 and also the 80 char cards that use 6845 that were sold for C64.

It could even have a socket for VIC-1 to make it VIC-20 comaptible, but
that would probably be a bit too much of a feature creep even for my
fantasy :)

If the board will have faster RAM, it might be a good idea to get rid
of the requirement to switch modes to let the 65** CPU or another CPU
use the RAM, instead let both run at the same time.

I don't know if the C128 CP/M uses Z80 opcodes or if it only uses
8080/8085 compatible code. If it isn't using Z80 opcodes, the board
could have a V20 instead of 8088 and use the 8080/8085 emulation mode
of V20 to be able to run the C128 CP/M mode.

(I think complete compatibility with C128 isn't worth the effort as it
would require compatibility with the strange 80 char chip on C128. But
I haven't looked into the possibility of using a 6845/6545/6345 with
some logic to emulate how C128 80 char mode works).

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